1. Field of the Invention
The present invention relates to a method of monitoring cathodic protection of an item located underwater at a facility and an underwater facility having an item requiring cathodic protection. In particular the invention is suitable for subsea hydrocarbon extraction facilities.
2. Description of Related Art
Metal items and surfaces which are deployed underwater, particularly subsea, are prone to corrosion, due to the electrolytic nature of the surrounding liquid. Such corrosion may result in underwater equipment failures with consequently significant costs of downtime and replacement.
A standard technique for reducing corrosion after deployment of underwater equipment is to use cathodic protection, commonly abbreviated to “CP”. A widely-used form of CP is “galvanic anode CP”, in which a sacrificial metal surface is positioned proximate to the metal item to be protected. A sacrificial metal material is chosen which has a greater magnitude electrochemical potential than the item to be protected. Commonly used sacrificial metal materials include alloys of zinc, magnesium and aluminium for example. When located subsea for example, the sacrificial metal material will be corroded preferentially to the item. Eventually, the sacrificial material will be corroded to such an extent that replacement of the sacrificial material is necessary.
The effectiveness of the CP, which is primarily dependent on the amount of corrodible sacrificial material remaining, may be monitored by measuring the potential between the metal item to be protected and the surrounding liquid, as is well-known in the art. For subsea CP for example, it is common to use silver chloride-based (AgCI) electrodes for this purpose. International standards are set which govern CP design, a current example being DNV-RP-B401. Currently, these standards recommend that effective corrosion protection of subsea equipment is not assumed until a potential of −0.8V (Ag|AgCl|seawater) across the system is attained. It is therefore necessary to monitor potentials on the equipment to ensure that the required level of CP is met.